Earth-boring bits of the rolling cone variety rely on the rolling movement of at least one cutter over the bottom of the bore hole for achieving drilling progress. The earth-disintegrating action of the rolling cone cutter is enhanced by providing the cutter with a plurality of protrusions or teeth. These teeth are generally of two types: milled teeth, formed from the material of the rolling cone; and inserts, formed of a hard material and attached to the rolling cone surface. Earth-boring bits of the fixed cutter variety, commonly referred to as drag bits, have no moving parts and employ an array of hard inserts to scrape and shear formation material as the bit is rotated in the borehole.
Until now, inserts on prior art fixed cutter bits have been aligned such that the inserts will scrape the material of the borehole bottom. For scraping to take place, the longitudinal axis of the insert is typically at a small acute angle, such as zero to 30 degrees relative to the bit face. Such an alignment places the cutting face of a cylindrically shaped insert nearly perpendicular to the borehole bottom. The contact area between the cutting element and the formation starts out to be very small but increases rapidly as penetration or depth of cut becomes deeper.
Cutting elements which have high initial aggressiveness are desirable in applications where weight on bit and torque are limited and a maximum rate of penetration for a given weight on bit is the goal. They can become a liability in applications where the weight on bit cannot be accurately controlled, as in directional drilling which causes harmful torsional oscillations. Commonly used approaches to reduce aggressiveness of directional bits include increased cutting face inclination (back rake), depth of cut limiters and increased cutting element density.
The high initial aggressiveness can also cause problems in hard, abrasive materials where the highly loaded tip of the cutting edge quickly breaks down and a large wear flat develops on the bottom side of the cutting element, bringing the much less wear resistant tungsten carbide backing into contact with the formation. This leads to accelerated wear and/or heat checking with subsequent catastrophic breakdown of the cutting element. Another shortcoming of the conventional cutting element shape and alignment is the flow of the drilled-up material or shavings, which is directed vertically upward into the face of the bit. On heavier set, high cutter density bits with small open face volume, it frequently leads to severe bit balling.